As one of cutting/grinding processing methods, there is a cutting/grinding processing method by minimal quantity lubrication system (also referred to as MQL, hereinbelow) in which a trace amount of oil ranging from a few mL per hour to a few tens of mL per hour is supplied together with compressed air to a metal to process the metal. Since MQL uses a less amount of oil than conventional metal processing methods, there is an advantage in reduction of the amount of wastes, and therefore, the range of applications of MQL is expanding as an environmentally friendly processing method.
In the case of MQL, even though only a trace amount of oil is supplied to processing sites, the influence of the oil is great. For instance, in the case where a water-soluble cutting fluid which has been used in conventional processing methods is used as is in MQL, the oil is ejected in the form of fine mist, and thus many of conventional water-soluble cutting fluids which contain an amine compound or the like may damage the respiratory system of operators.
Meanwhile, MQL is also required to have processability comparable to or higher than that of conventional processing methods. However, since there is a fundamental difference in working mechanism between MQL and conventional methods, if oil used in conventional processing methods is applied to MQL as is, the processability tends to be insufficient. For this reason, oils based on vegetable oil and synthetic ester suitable for MQL have been developed and widely used (for example, see Patent Literatures 1 to 4).